A coherent receiver module is used in a transmission system adopting dual polarization and quadrature phase-shifted keying modulation format (DP-QPSK). By transmitting 2 bits per symbol on orthogonal polarizations this modulation format quadruples the capacity of a simple on-off keying systems and also extends transmission reach at the same time. The function of the coherent receiver is to demodulate the transmitted waveforms by separating the signal (Sig) into two orthogonal polarizations (SigX and SigY) and resolving the phase of each polarization into in-phase (I) and quadrature-phase (Q) components by coherently mixing the separated signals with a Local Oscillator (LO). A critical aspect of this receiving process is that the SigX and SigY should have a fixed phase relation to the LO; and it is also important to balance both the time delays and the field amplitudes at photodetectors where the fields are mixed. A further complication of this demodulation process is that balanced detection must also be used to suppress common mode noise that would otherwise corrupt the signal quality of the I and Q components. Moreover, all these requirements must be satisfied over a wide operating wavelength range, high symbol rate, and over an extended temperature range
Illustrated in FIG. 1 is the general configuration of the 90° optical hybrid that is needed to resolve a signal into I and Q components. Here the phase relationship between the signal amplitude S (either SigX or SigY) and co-polarised LO amplitude L is shown: the S−L and S+L indicate that the two outputs from the hybrid have a phase difference of 180° between each other, and form the I channel while the S−jL and S+jL indicate that the two outputs have a phase relationship displaced by 90° with respect to S−L and S+L, and form the Q channel. While the time/phase difference, or skew, between the I and Q channels of both SigX and SigY is of concern, the most exacting requirement is reserved for the difference in time delay between the balanced detectors from which the I and Q components are derived. This latter requirement places stringent demands upon the technologies from which the coherent receiver is constructed.
The 90° hybrid can be realized with either a waveguide mode interference coupler (FIG. 2A) or a 4×4 Multimode Interference (MMI) coupler (FIG. 2B). While the former is usually used in a material system with lower refractive index contrast, such as Silica, the latter is more appropriate for a material system with high refractive index contrast, such as Indium Phosphide (InP) or Silicon-On-Insulator (SOD. In the case of a 4×4 MMI coupler, the SigX or SigY and LO are launched into the optical 90° hybrid asymmetrically (using either ports 1, 3 or ports 2, 4) to construct a 180° phase difference between the four output waveguides with the middle two outputs (WG2 and WG3) as a pair and the two outermost outputs (WG1 and WG4) as another pair which need to be brought together to the differential input of the Trans-Impedance Amplifier (TIA) by either two waveguide crossings (FIG. 2B) or electrical crossings (FIG. 2C).
A fully integrated DP-QPSK receiver chip should contain all elements shown in FIG. 3, including                (1) two pairs of four high speed (32Gbaud) photodetectors        (2) two 90° optical hybrids        (3) one optical signal input and two pairs of four electrical outputs        (4) one 1:2 Beam Splitter (BS)        (5) one narrow linewidth tunable laser (LO)        (6) one Polarization Beam Splitter (PBS)        (7) one Variable Optical Attenuation (VOA), and        (8) one Monitor (MON)        
As is seen in FIG. 3, the components in (1)-(3) are configured into two nominally identical sub-circuits which are used to mix the signal polarizations SigX, SigY which are separated by (6), with a local oscillator (5), which is first split by (4). Also included are a variable optical attenuator (7) and monitor (8) which are used to condition the signal prior to demodulation.
Due to the complexity of the functionalities and the stringent performance requirements on the device, the fully integrated solution has always been the first choice [U.S. Pat. No. 0,054,761 A1, 2010, Y. K Chen, et al.], however extensive development has also been seen on hybrid solutions, i.e. a combination of integrated solutions for a subset of the items in the above list, and the rest being discrete components. The former wins on the reliability and compactness at the cost of yield and manufacturability because of the integration of several extremely difficult components; while the latter wins on the opportunity to combine known good components with best available performance at the cost of yield and manufacturability because of a more complicated and high precision assembly processes.
In the prior art, there are different hybrid combination solutions, depending on the chosen technology and technical approaches. The three most common implementations are:                1) One integrated chip incorporating elements (2)-(4) and (6)-(7), where (2) uses two waveguide mode interference couplers, is assembled with a high speed photodetector array and an external LO [S, Tsunashima, F. Nakajima, Y. Nasu, R. Kasahara, Y. Nakanishi, T. Saida, T. Yamada, K. Sano, T. Hashimoto, H. Fukuyama, H. Nosaka, and K, Murata, ‘Silica-based, compact and variable-optical-attenuator integrated coherent receiver with stable optoelectronic coupling system’, Optics Express, Vol. 20, p 27174, 2012]. The weakness of this approach is that it places the stringent requirements on the optical assembly processes in order to meet the performance requirements on the in-channel skew and field amplitudes balance.        2) Two coherent receiver chips, which contain the items (1)-(3), where (2) uses two 4×4 MMI couplers with output waveguide crossings [A. Belling, N. Ebel, A. Matiss, and G. Unterborsch. ‘Fully-Integrated Polarization-Diversity Coherent Receiver Module for 100 G DP-QPSK”, OML5, OFC '2011], as shown in FIG. 4. This approach has the 90° hybrids and photodetectors monolithically integrated on the same chip; and while this arrangement addresses the danger of performance impairments caused by skew between the I and Q channels and between detector pairs, the chip suffers from both a large footprint and high optical loss. These disadvantages are the results of long propagation path required to realize low loss and low crosstalk optical waveguide crossings that are necessary for this layout. A further disadvantage of this arrangement is that two die attachments with simultaneous optical and polarization alignments are needed on the four optical input ports—a difficult and unreliable assembly process. Also, because SigX and SigY are derived from separated optical assemblies skew between polarization components is also of concern.        3) One coherent receiver chip, which contains items (1)-(4), where (2) uses two 4×4 MMI couplers as optical hybrids, together with electrical crossings over the optical waveguides, as shown in FIG. 5 (V. Houtsma, N. G. Weimann, T. Hu, etc. ‘Manufactural Monolithically Integrated InP Dual-Port Coherent Receiver for 100 G PDM-QPSK Applications’, OML2, OFC'2011). This configuration is very compact because of the higher integration level and vertical electrical-optical crossings; and it also requires only a two port optical alignment. However, it still poses three issues: first is that the LO port is sandwiched between the two signal ports and so it complicates optical assembly; second, the electrical outputs are not co-linear with the optical inputs, creating further difficulty in module design; and third, the output waveguide path length is not balanced which results in large channel skew.        